Easy way to get all 3 Caravan and Hi-Score Mode Achievements on the very 1st level.
In one life proceed through the level rescuing the prisoners until you get to the incline that leads to the boss.
Near the top of the incline are some water falls that if you shoot the middle and right one a lot of times reveals 4 hidden prisoners.
Continue to the end of the level rescuing the remaining two prisoners brining the total to 10 and finish the level in the Metal Slug should net you 170k by the end of the 1st mission.